Image formation in electrophotography is performed according to the following procedure. First, an electrostatic latent image is formed on a photoreceptor by employing a photoconductive material and then, the latent image on the photoreceptor is developed by use of a toner to form a toner image. Subsequently, the toner image on the photoreceptor is transferred onto a transfer material such as paper and thereafter, fixing is performed by employing heat, pressure or solvent vapor to prepare a print. Further, a residual toner on the photoreceptor is cleaned by various means. Image formation is thus performed through the foregoing steps.
Recently, such image forming apparatuses have been employed not only as a copying machine to copy original manuscript but also as a printer to output image data outputted by an office computer or in the field of personal computers. Further reduction in size and weight, enhanced speed and reliability have been severely required from such a background, and machines have been constituted of simpler elements from various points. Meanwhile, performance required for a toner has become higher and it is in such a situation that the foregoing requirements are not fulfilled without attainment of toner performance.
Performance required for a toner include a variety of characteristics such as an electrostatic-charging property and a fixing characteristic. Specifically, a binder resin constituting a toner is required for enhancements of fixing performance onto transfer paper and offset resistance performance. Herein, the offset resistance performance refers to the capability of not allowing a toner to cause cold offset or hot offset at a given temperature, as described below.
In heat roll fixing, for instance, toner particles electrostatic-attached onto transfer paper are passed between heated pressure rolls, melted and fixed onto the transfer paper. However, when the surface temperature of a roll is too low, the whole of a toner particle layer is not sufficiently heated and only the surface portion in contact with the heated roll is softened and adhered to the roll. Any toner on the transfer paper side is not softened, causing no adhesive force, so that the toner layer on the transfer paper is transferred onto the fixing roll side instead of being fixed onto the transfer paper. Such phenomenon is called cold offset.
On the contrary, when the surface temperature of a roll is too high, the viscosity of the melted toner is lowered. Accordingly, the internal cohesive force of the melted toner layer is rapidly lowered, resulting in reduced adhesion onto the heated roll. As a result, the melted toner layer is ruptured and transferred onto both the transfer paper and the fixing roll. Such a phenomenon is called hot offset, often causing adverse staining of the heating roll. A toner adhered to the heated roll is again transferred onto transfer paper and tends to stain non-imaging areas, resulting in lowering of image quality.
Thus, a binder resin constituting a toner is required to exhibit offset resistance performance and not causing cold offset or hot offset within a broad temperature range and also to exhibit superior fixing characteristics.
Further, acceleration of image formation is desired at least in the field of color copiers and color printers. Rapid fixability is required for such acceleration and there is subsequently also desired a toner providing both rapid fixability and low temperature fixability as toner performance.
There were proposed techniques to improve offset resistance, such as addition of an offset inhibitor, and designation of a binder resin with a low molecular weight polymer component and a high molecular weight polymer component. These techniques overcame problems in offset resistance but there still remained problems in compatibility with low temperature fixing, such as an elevated softening point.
There was also proposed a technique in which a binder resin was formed of a styrene-acrylic acid polymer, the molecular weight distribution of such a polymer was broadened without having a high-molecular weight region and a cross-linking structure due to ionic bonding between carboxyl groups of the polymer was formed by use of a metal compound. Such a technique was expected to achieve a substantially increased molecular weight and realize enhanced offset resistance, as described in, for example, JP-A No. 110156 (hereinafter, the term JP-A refers to Japanese Patent Application Publication). However, such a toner used for development of electrostatic images was composed of a large amount of a metal compound, so that such a metal compound exhibited catalytic action with depending on conditions, resulting in tendency of the resin in the toner being easily gelled. Accordingly, offset resistance was enhanced but there were problems such as deteriorated low temperature fixability.
There was also known the use of a high-acid value polyester resin used for a binder resin, which was targeted for low temperature fixing. There was proposed, for example, a toner containing a polyester resin having a prescribed acid value, hydroxy group value, molecular weight distribution and tetrahydrofuran-insoluble component, as described in, for example, JP-A No. 9-204071. However, such a proposal simultaneously resulted in lowering of low temperature fixability and a lowered melting temperature tended to deteriorate offset resistance. To realize a toner compatible in low temperature fixability and offset resistance was required a further extensive study.
There have been studied methods of introducing a cross-linking component or a polymeric component into a binder resin to achieve an expanded fixing temperature range or enhanced offset resistance in the prior art. However, a technique for introduction of a cross-linking component made it difficult to control cross-linking density. Specifically, an emulsion association method in which a sub-micron resin particle dispersion was mixed with a colorant dispersion to prepare toner particles through coagulation and fusion made it difficult to control the particle shape. On the other hand, a method of introducing a polymeric component required excessive thermal energy to control the particle shape, due to increased viscosity of the binder resin and also resulted in increased melt viscosity during fixing, which became disadvantageous conditions for achievement of low temperature fixing.